The present invention pertains to a method and apparatus for forming a continuous chain of stitches between pieces of material on a sewing machine having two needles and a single looper located beneath the needle plate.
In sewing machines of this type the distance between the needles can vary, as is well known, in accordance with the particular stitching operation for which a machine is intended and the distance may vary as much as, or even exceed 6 millimeters.
More precisely, when the distance between needles is around 4 millimeters, it is not possible to form a continuous chain of stitches between pieces of material due to the threads of the needles having too great a distance between them in relation to the tension to which they are subjected by the looper thread, and because the needle plate in this type of machine has needle holes that extend in the direction of movement of the material and are located directly in the path of the teeth of the feed dog.
Consequently, when the stitches are no longer formed on the material, the traction action of the looper and the feed dog ceases on the threads of the needles. This traction action provides the required means for the correct linking of the threads beneath the needle plate. It is considered advantageous to form a chain of stitches between pieces of material sewn on the automatic machines described above without intervention of an operator even when the distance between the needles exceeds 4 millimeters so as to eliminate the necessity of pulling the threads manually to effect the linking together thereof.
Additionally, with no means for effecting a continuous chain of stitches between pieces of material, the conventional chain cutting devices cannot be utilized for cutting the seam threads adjacent the front and rear edges of the material and requires this severing operation to be performed manually in successive stages.
The method of forming a continuous chain of stitches according to the invention can be accomplished automatically in sewing machines having two needles regardless of the distance therebetween and with a single looper beneath the needle plate. The method also permits utilization of conventional chain cutting devices and reduces the number of additional handling and finishing operations that must be performed on the sewn material.
By means of this method, the tension on the thread of one of the needles is released when a stitched piece of material is advanced beyond the sewing area and this thread is maintained adjacent the tensioned thread of the other needle beneath the needle plate of the machine. This is effected by the relative movement, one toward the other, of the tensioned thread of the other needle and of the looper which remain unchanged so as to form a single chain of stitches.
The apparatus for accomplishing the method described above includes a device for releasing the tension of the thread of one of the needles which is operatively connected to a sensing means that is actuated by the lack of material in the sewing area. An element is also provided that forms an integral part of the needle plate and which extends transversely to the path of travel of the chain of stitches and serves to support said chain.
Additionally, the apparatus includes an auxiliary feed member that is supported by the machine's pressor foot and cooperates with the above needle plate element and feed dog, in the performance of its intended function to effect forward movement of the chain of stitches.
These and other features of the invention will become more fully apparent by reference to the appended claims and as the following detailed description proceeds in reference to the figures of drawing wherein: